The present invention relates to an optical multiplexing/demultiplexing device of the power distributing type for use in multiplexing and demultiplexing optical fiber communication circuits. In particular, it relates to an optical multiplexing/demultiplexing device for multiplexing an optical signal into four or more optical signals, or demultiplexing four or more optical signals into a single output signal.
Optical multiplexing/demultiplexing devices with multiple ports, which are used for multiplexing or demultiplexing optical signals in optical fiber communication circuits, have conventionally been made by scrubbing the optical fibers, thermally alloying the optical fibers, and by using optical waveguides.
FIGS. 3(A) and 3(B) show a conventional optical multiplexing/demultiplexing device which is made by scrubbing optical fibers. FIG. 3(A) shows a cross-sectional view cut across the optical axes of the optical fibers. FIG. 3(B) shows a cross-sectional view cut along the optical axes of the optical fibers.
The conventional optical multiplexing/demultiplexing device will be described hereinafter.
V-grooves 23 and 24 are fabricated by manual work at the centers of a pair of substrates 21 and 22. Optical fibers 25 and 26 are put into V-grooves 23 and 24 and fixed there by an adhesive agent. Thereafter, clads 29 and 30 of the optical fibers are scrubbed together with substrates 21 and 22 in parallel with the optical axes thereof until cores 27 and 28 of the optical fibers are just exposed. Substrates 21 and 22 are then assembled together with optical fibers 25 and 26 to form the optical multiplexing/demultiplexing device so that the scrubbed surfaces of optical fibers 25 and 26 mate each other on a pair of substrates 21 and 22.
With the above-mentioned structure, in the 2.times.2 optical multiplexing/demultiplexing device shown in FIGS. 3(A) and 3(B), almost all the optical input P.sub.i from one end of the optical fiber 25 is transmitted within the core 27. However, there exists a leakage of light from the boundary of core 27 and clad 29, which forms an Evanescent field, most of this light leakage occurring near the core portion. By scrubbing off a surface of the clad 30 of optical fiber 26 and the clad 29 of optical fiber 25, the distance S between the cores 27 and 28 may be selected. Also, as shown in FIG. 3(B), the contacting length L of fibers 25 and 26 may be selected.
By varying the distance S and the length L, the amount of photocoupling of optical fibers 25 and 26 generated through the Evanescent field may be changed. By adjusting the amount of photocoupling, the quantity of light transmitted from optical fiber 25 to fiber 26 may be adjusted. Thus the input light P.sub.i from the optical fiber 25 may be multiplexed to output powers P.sub.01 and P.sub.02, with P.sub.01 and P.sub.02 having a predetermined ratio. This type of optical multiplexing/demultiplexing device is of the power distributing type, and therefore may be used for power distribution.
The Evanescent field has a so-called wavelength dependent nature in which the spreading pattern of light changes according to the wavelength of the light. By utilizing this phenomena and by appropriately selecting the distance S between the cores and the contacting length L, a wavelength-divided type multiplexing/demultiplexing device may be formed, in which light having multiple wavelengths, consisting of a plurality of different optical wavelengths, may be wavelength-divided to branch into a plurality of outputs.
In the power distributing type of optical multiplexing/demultiplexing device, in which an input signal P.sub.i of a single optical wavelength of 1300 nanometer is branched into eight optical output ports P1 to P8, seven 2.times.2 multiplexing/demultiplexing circuits are used, each of the circuits being designed to have a 50:50 multiplexing ratio.
Optical fibers 31 and 32 of optical multiplexing/demultiplexing unit A consisting of a 2.times.2 circuit are connected to optical fibers 33 and 34 of optical multiplexing/demultiplexing units B and C, each consisting of a 2.times.2 circuit. Optical fibers 35 and 36 of optical multiplexing/demultiplexing unit B consisting of a 2.times.2 circuit are connected to optical fibers 39 and 40 of optical multiplexing/demultiplexing units D and E, each consisting of a 2.times.2 circuit. Optical fibers 37 and 38 of optical multiplexing/demultiplexing unit C consisting of a 2.times.2 circuit are connected to optical fibers 41 and 42 of optical multiplexing/demultiplexing units F and G, each consisting of a 2.times.2 circuit. Connections between two optical fibers are actualized by using optical fiber connectors labelled 43, or by means of arc discharges.
Internal optical fiber connections require additional assembling, scrubbing, and testing costs. Welding of optical fibers by means of arc discharges degrades the optical performance of the device due to its abrupt heat-cool cycle and thus the optical power losses at the respective internal connection junctions are accumulated in the optical fiber transmission system.
Multiplexing of four to 12 core optical fibers, which is being put into practice, requires a greater number of connection points. For instance, the number of connection points for demultiplexing the optical signals into eight core optical fibers is calculated to be 48 points as shown in FIG. 4, and thus a compact, light-weight version is difficult to be actualized.
An objective of this invention is therefore to provide an optical multiplexing/demultiplexing device of a compact, light-weight type with reduced power losses at its internal connection junctions, reducing the number interconnections among the units consisting of one or more components, each of which consists of a pair of substrates and a pair of optical fibers in the system configuration, where an optical fiber can pass through a single unit consisting of a plurality of components connected in series to multiplex/demultiplex the optical signals.